The present invention relates to wireless communications, and more particularly to the sensing of wireless transmissions from a licensed user of a licensed spectral resource.
The radio spectrum is a limited resource that should be shared between many different types of equipment such as cellular, home network, broadcast, and military communication equipment. Historically, each part of the radio spectrum has been allocated to a certain use (called a “licensed” and/or “primary” use). This strategy has resulted in all applications/uses being disallowed on the allocated carrier frequency except for those applications included in the license agreement. In practice, this results in large parts of the radio spectrum being unused much of the time. For instance, in the Ultra-High Frequency (UHF) band, where TV broadcasts take place, large geographical areas are unused, mainly due to the large output power needed for such applications; this large output power compels a large reuse distance in order to minimize the risk of interference. An example of such geographical areas within Scandinavia is illustrated in FIG. 1. In FIG. 1, the shaded areas represent geographic locations in which a given carrier frequency is being used by a licensed user (e.g., by Broadcast TV). In the remaining areas, the so-called “white spaces”, the given carrier frequency is allocated to the licensed user but is not actually being used by that user.
In order to make better use of the licensed spectral resources, some countries will, in the future, allow unlicensed services (so called “secondary” uses) to take place in areas (called “white spaces”) in which the licensed (primary) user is not transmitting. However the primary user will always have priority for the use of the spectrum to the exclusion of others. Therefore, some kind of sensing mechanism is needed in the (unlicensed) devices to enable them to detect whether a licensed user is currently transmitting. If such licensed use is occurring, the unlicensed user needs to turn off its transmission on that carrier frequency. The most straightforward sensor is a signature detector adapted to detect specific signatures transmitted from the licensed/primary user (typically implemented as a matched filer).
Another consideration regarding the sensing of the licensed user's transmissions is placement of the sensors. When the secondary (i.e., unlicensed) use is for cellular telecommunications, one solution is to include the sensors in the base station of the mobile communication system. However due to shadow fading and the like, there is a risk that the base station will not detect the primary user's transmissions despite the fact that a mobile device/terminal (generally referred to as “user equipment”—“UE”) (connected to base station) is (or is capable of) interfering with the licensed user. One way to combat this problem is to include a sensing device in all UEs operating on these white space carriers; that is, all UEs can detect licensed use of the spectral resources and respond by turning off their own unlicensed transmissions. A main problem with sensing is that there are requirements to detect a licensed user's activity rather quickly, implying that sensing needs to be performed quite frequently. Since sensing typically includes reception and decoding of a radio signal, the more frequently a UE performs detection, the more power it consumes. This is an undesirable consequence, especially for battery-operated devices.
Therefore, there is a need for systems capable of operating in white spaces in an efficient manner that seeks to reduce the cost (in terms of energy and otherwise) and complexity involved in such unlicensed operation, while still having good performance in connection with detection of potential transmissions of the primary user.